1. Field of the Invention
The present invention has to do with additives used in the wet end of the papermaking processes. More particularly, the invention relates to swollen starches which are added as dry strength additives in the papermaking process.
2. Description of Related Art
Paper is manufactured for the most part from wood pulp. There are many different kinds of wood pulp: mechanical pulp (ground wood), semi-chemical pulp, sulfite pulp, sulfate or kraft pulp, soda pulp, and the like. Some are prepared by purely mechanical means, others by a combination of mechanical and chemical means, and some by chemical means. The mechanical pulp contains substantially all the wood except the bark and that lost during storage and transportation. Semi-chemical pulps are partially free of lignin. Chemical pulp, however, are essentially cellulose, the unwanted lignin and other non-cellulosic components of the wood having been dissolved away by the cooking and bleaching treatment. Because of this, chemical pulps are much superior to mechanical and semi-chemical pulps for fine papermaking. However, because of the special processing required, they are too expensive to serve as a main source of fiber for the cheaper grades of paper such as newsprint.
If the pulp fibers were the only constituents of a paper sheet, the usefulness of the paper would be very restricted because the sheet would be soft, have a yellowish color, and could not be successfully written or printed upon with ink. If the sheet was thin, it would be transparent to matter printed upon the opposite side. It is necessary, then, to add other ingredients, such as sizing or coloring agents and fillers to the cellulosic fibers to produce papers suited to their many uses.
Many papers, except the absorbent types, filter papers and most packaging papers, must have a finely ground filler added to them, the purpose of which is to provide opacity and whiteness to the paper. Sizing is added to the paper, other than absorbent papers and filter paper, to impart resistance to penetration by liquids. The common sizing agents are added to the stock (i.e., wet pulp, more specifically, the prepared papermaking furnish) before it is formed into a sheet. These may be acid, neutral and alkaline sizing materials. Acid sizes are typically rosin based and precipitated by alum. Neutral sizes may also be rosin-based, but are used at near neutral pHs. Alkaline sizes are synthetic materials such as alkenyl succinic anhydride (ASA) and alkyl ketene dimer (AKD). Such sizings as described in this paragraph are known in the trade as internal sizing.
The term sizing is also used in a second context in the paper industry. This second use is known as surface sizing. It differs from the internal sizing previously described in that it is applied to the surface of the paper where it cements the fibers to the body of the paper and deposits a more or less continuous film on the paper surface. Surface sizing is used to produce a smooth hard surface which will not catch a pen when the paper is written upon, will not pick off if the paper is printed using tacky inks and will not show feathering of the ink. An additional advantage of a surface size is that oil resistance of the paper is improved since the size tends to seal the pores of the paper. Surface sizing may be of greater importance than internal sizing for certain types of papers such as writing papers, printing papers and some wrapping papers. It is important to surface size paper that is used in an offset printing process since this prevents loosening of surface fibers when the paper is moistened with water on the press.
A common way to add a surface size is to apply the sizing agent to both sides of the paper at the size press. Inherent in this process are various mechanical problems which make it expensive to apply and maintain a uniform layer of the size on the surface of the paper. This mechanical equipment is costly, and there is the added expense of evaporating the water added to the paper with a dilute suspension of the size. Commonly the size is a starch or a starch derivative such as an oxidized or enzyme converted starch.
Starch has long been used as an additive to strengthen the paper sheet in the manufacture of paper. See for example Whistler and Paschall, Starch: Chemistry and Technology, Academic Press Inc., New York, N.Y. Vol. 2, 1967, Chapter VI. For this purpose, starch is added to the stock. This procedure produces much soluble material which is not effectively retained in the paper sheet. An improvement in starch cooking was disclosed in U.S. Pat. No. 2,805,966, issued Sep. 10, 1957, wherein the starch slurry was heated in a steam injection cooker. This was said to permit control of the heating so that the majority of the starch granules were swollen but not ruptured. However, the temperature range over which granules of starch swell and gelatinize is large. Even in this process only a portion of the granules could be obtained in the desired swollen state. Some starch granules were still unswollen and hence useless as an adhesive while others might have been solubilized and not retained in the paper.
One method to obtain a starch whose swollen granules do not disintegrate during agitation was disclosed in U.S. Pat. No. 2,113,034, issued Apr. 5, 1938. This was accomplished by reaction of starch with formaldehyde. The product is resistant to dispersion in hot water, and requires treatment with alkali and vigorous agitation to render the starch suitable as a paper stock additive. As a result of these treatment requirements and because the starch is only partially retained in the paper sheet, the product has never found acceptance in the paper manufacturing industry.
A second method to obtain a starch whose swollen granules would not disintegrate during agitation was disclosed in U.S. Pat. No. 2,328,537, issued Sep. 7, 1943. This was accomplished by reacting the starch with certain antimony or phosphorous chlorides or oxychlorides. The patent suggested that the products might be useful in the manufacture of paper. Again, these products have never found acceptance in the paper manufacturing industry, because the products described show limited swelling in hot water and are only partially retained in the paper sheet.
It would therefore be an advance in the art if an additive were discovered which could be incorporated into the paper stock prior to and/or during the sheet formation that would impart improved internal properties to the paper. If such properties could be imparted without causing detrimental side effects, the additive in question would find ready acceptance in the art. Moreover, if the properties of the finally formed paper article, such as burst strength and tensile strength were improved by means of the additive, this would be an added economic benefit to be gained through use of an additive of this type.
It is therefore an object of the invention to provide a new and improved method for imparting such characteristics to manufactured paper by the addition of a specific additive to the stock during the papermaking process.
A further object is to provide an agent of this type to improve properties without interfering with the other additives and substances used in the makeup and manufacture of paper and without showing adverse effects on the chemical and physical characteristics of the finished sheet.
Another object of the invention is to provide an additive for improving internal properties which is retained in the paper during the sheet forming process.
An important object of the invention is to provide an additive for improving properties of manufactured paper which will operate on a wide variety of paper stocks, is safe to handle and will impart to the finished sheet desirable characteristics.
Another object of this invention is to provide a method of improving surface properties of manufactured paper by employing a new additive at the wet end which imparts improved dry strength to the finished paper article.
A still further object of this invention is to provide a starch additive for paper that is readily prepared and whose swollen granules do not disintegrate when subjected to vigorous agitation.
The term "wet end" as it is used herein refers to that part of the papermaking process including the stock preparation, the approach flow system and that part of the paper machine prior to the dryer section.
All percentages set forth herein are by weight unless otherwise specified.